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Vacuum Evaporation

Vacuum Evaporation. Lecture 8 G.J. Mankey gmankey@mint.ua.edu. Monolayer Time. The monolayer time is the time for one atomic layer to adsorb on the surface: t = 1 / (SZA).

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Vacuum Evaporation

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  1. Vacuum Evaporation Lecture 8 G.J. Mankey gmankey@mint.ua.edu

  2. Monolayer Time • The monolayer time is the time for one atomic layer to adsorb on the surface: t = 1 / (SZA). • At 3 x 10-5 Torr, it takes about one second for a monolayer to adsorb on a surface assuming a sticking coefficient, S = 1. • At 10-9 Torr, it takes 1 hour to form a monolayer for S = 1. • For metals at room temperature S = 1, so the vapor pressure should be >10-6 Torr. Sticking Coefficient S = # adsorbed / # incident Impingement rate for air: Z = 3 x 1020P(Torr) cm-2 s-1 Area of an adsorption site: A » 1 Å2 = 10-16 cm2

  3. Vapor Pressure Curves • The vapor pressures of most materials follow an Arrhenius equation behavior: PVAP = P0 exp(-EA/kT). • Most metals must be heated to temperatures well above 1000 K to achieve an appreciable vapor pressure. • For PVAP = 10-4 mbar, the deposition rate is approximately 10 Å / sec.

  4. Physical Evaporation Substrate • A current, I, is passed through the metal boat to heat it. • The heating power is I2R, where R is the electrical resistance of the boat (typically a few ohms). • For boats made of refractory metals (W, Mo, or Ta) temperatures exceeding 2000º C can be achieved. • Materials which alloy with the boat material cannot be evaporated using this method. Flux Evaporant Boat High Current Source

  5. Limitation of Physical Evaporation • Most transition metals, TM, form eutectics with refractory materials. • The vapor pressure curves show that they must be heated to near their melting points. • Once a eutectic is formed, the boat melts and the heating current is interrupted.

  6. Electron Beam Evaporator Substrate • The e-gun produces a beam of electrons with 15 keV kinetic energy and at a variable current of up to 100 mA. • The electron beam is deflected 270º by a magnetic field, B. • The heating power delivered to a small (~5mm) spot in the evaporant is 15 kV x 100 mA = 1.5 kW. • The power is sufficient to heat most materials to over 1000 ºC. • Heating power is adjusted by controlling the electron current. e-beam Flux Evaporant Crucible B cooling e-gun

  7. Wire Evaporator substrate • This is a "mini" version of the electron beam evaporator. • The entire assembly fits through a 2 3/4 " OD Flange. • Electrons from the heated filament bombard a 2 mm wire that is held at a large positive bias. • The power supply is operated in a current limiting mode and the heating power is P = VbiasIemission. filament cooling shroud 1-2 kV 0-12V

  8. Wire Basket • Direct or alternating current is passed through a pre-fabricated helical wire container. • Evaporant placed in the helix is heated by contact and irradiation. • Heating power is of the order of 100 W or more with a refractory helix with 0.1 - 0.5 mm diameter wire. • Works for Ag, Au, Cu, Cr, Mn, etc. cooling shroud 1-20 V

  9. Knudsen Cell • The crucible is heated by a coil or heater surrounding it. • Crucibles are usually made of boron nitride, alumina, or graphite. • Since there is a large amount of heat, the device is constructed of low outgassing materials and a large amount of cooling is necessary. cooling shroud 1-20 V

  10. ? Measuring and Calibrating Flux • Many fundamental physical properties are sensitive to film thickness. • In situ probes which are implemented in the vacuum system include a quartz crystal microbalance, BA gauge, quadrupole mass spectrometer, Auger / XPS, and RHEED. • Ex situ probes which measure film thickness outside the vacuum system include the stylus profilometer, spectroscopic ellipsometer, and x-ray diffractometer. • Measuring film thickness with sub-angstrom precision is possible.

  11. Frequency Measurement Conversion to Thickness Display Substrate Quartz Crystal Flux Quartz Crystal Microbalance • The microbalance measures a shift in resonant frequency of a vibrating quartz crystal with a precision of 1 part in 106. • fr = 1/2p sqrt(k/m) » f0(1-Dm/2m). • For a 6 MHz crystal disk, 1 cm in diameter this corresponds to a change in mass of several nanograms. • d = m / (rA), so for a typical metal d » 10 ng / (10 g/cm3*1 cm2) = 0.1 Angstroms.

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